1. Field of the Invention
The present invention relates generally to fluid dynamometers, and more particularly to a fluid dynamometer that adjusts the characteristics of the fluid contained therein to thereby adjust the power absorbing characteristics of the dynamometer.
2. Description of the Prior Art
Dynamometers are devices which are used to measure and absorb the power output of engines or power producing devices. This output is usually transmitted by a rotating shaft to the dynamometer. There are two basic types of dynamometers, a brake dynamometer and a disk dynamometer. In a brake type dynamometer a frictional load is imposed upon the rotating shaft to absorb the energy produced. The amount of force needed to impose the frictional load is equatable to the power being produced by the engine.
In a disk dynamometer, a disk is affixed to the rotating shaft of the engine. The disk is submerged in a container of fluid, typically tap or fresh water. The power produced by the engine is absorbed by the well known interaction of the disk and fluid at the disk/fluid interface. In this type of dynamometer, there often are no physical or mechanical measurements of torque or force caused by the rotating shaft. Rather, the disk is calibrated to absorb a specified amount of power in fresh water at 68.degree. F. for a prescribed number of shaft revolutions per minute (RPM). In this way, one may determine power or energy output of the engine by measuring shaft RPM.
In addition to single disk dynamometers, there are multiple disk and counter-rotating disk dynamometers. For multiple disk type dynamometers, the dynamometer tank housing usually includes a number of projections interspersed between the rotating disks to assure that a cylindrical water slug is not formed about the rotating components. The formation of a water slug would destroy or cause a variation in the known power absorption characteristic of the disk assembly. The counter-rotating disk type dynamometer is used to measure total engine power output when the engine has two power output shafts which rotate in different directions.
For single or multiple disk dynamometers, the equation relating a disk's size, shaft rotational speed and horsepower is usually in the form: EQU HP=1.78.times.10.sup.-11 CR.sup.4.6 .rho..nu..sup.1/5 .omega..sup.2.8
where HP is the shaft horsepower;
.rho. is the density of the fluid in slugs/ft.sup.3 ;
C is a dimensionless constant verified by a horsepower calculation that uses shaft torque and angular velocity measurements;
.nu. is the kinematic viscosity of the fluid media in ft.sup.2 /sec;
R is the disk radius in inches; and
.omega. is the angular velocity of the disk in revolutions per minute (RPM). For multiple and counter-rotating disk dynamometers, the horsepower equation is evaluated for each of the disks.
From the foregoing relation, it can be seen that to absorb a different amount of power at a given rotational speed under standard test conditions (i.e., water at 68.degree. F.), the disk radius is the only parameter which can be modified. Modification of the "C" parameter, or constant, is not considered because it relates primarily to the dynamometer tank geometry. The standard practice is to optimally size the disk's diameter to absorb the power output of the engine being tested. Therefore, it is necessary to have a number of disks available, each uniquely suited to absorb a specified engine output as a function of rotational speed. If the disks available are unsuited to absorb the specified power, an additional disk must be made, or an existing disk modified for the application. Providing additional disks and/or modifying existing disks is both costly and time consuming. Another method of adjusting the load is to partially fill the tank or vary the amount of water in the tank. However, drawing fluid from the tank results in an air-water interface that produces system oscillations, unsteady conditions and noise.